There has been a flash memory used for a backup of data or the like.
Once data is written into the flash memory, it is impossible to overwrite data in units of pages. In addition to this, when data is rewritten, it is necessary to write changed data again after collectively erasing data in units called blocks. The flash memory has such a characteristic as described above. In addition, there is a restriction of the guaranteed number of times a block is erased (for example, about a million times), and it is noted that when the number of times a block is erased exceeds the guaranteed number, a case may occur in which the block is changed to a block, called a defective block, in which the value of written data is not ensured.
In addition, in some cases, when a flash memory is manufactured and completed, a defective block exists in the flash memory owing to the manufacturing process thereof or the like.
As a method for managing a defective block, there has been a method in which block state management means for managing a defective block in the flash memory have been stored in the flash memory and these block state management means are generated on memory means including a volatile memory, at the time of system initialization after power activation.
Here, when the block state management means stored in the flash memory is erased for any reason or when the storage point of the block state management means stored in the flash memory is lost, a processing operation for creating the block state management means again is performed.
However, for example, if information stored in all blocks of the flash memory is erased once and a procedure for rereading a defective block or the like is performed, so as to identify the defective block, at the time of the re-creation of the block state management means, a case may occur in which information for identifying the defective block disappears from the flash memory. In this case, a position that is primarily a defective block seems to be a valid block, and a problem occurs in which the reliability of data stored in the flash memory after that is reduced.
In addition, while the flash memory has been described, the same problem occurs in another semiconductor memory in which data can be rewritten and electrically erased.
An example of the related art is Japanese Laid-open Patent Publication No. 2002-32256.